1. Field of the Invention
This invention relates generally to the curing of retreads upon tire carcasses, and more specifically, to controlling the shrinkage of retreads during the tire curing process.
2. Description of the Related Art
It is commonly known to form a retreaded tire by attaching a pre-cured tread to a tire carcass. The tread is typically pre-formed into a strip having a tread pattern, and is later applied about the circumference of a prepared tire carcass. The tire carcass is typically buffed substantially free of the old tread, and a layer of cushion gum or liaison rubber is applied to the buffed tread area prior to receiving the new tread.
Once the retreaded tire is assembled, a flexible curing membrane is placed about the tire to retain the tread in proper position relative to the tire carcass. The membrane-laden retreaded tire is then placed into a pressure chamber within a curing vessel, such as an autoclave, to cure the tread to the tire carcass. When the curing cycle begins, substantially all of the air contained within a compartment formed between the tire and the membrane has been removed by a vacuum source. Accordingly, the flexible membrane is forced against the tread and tire carcass. This compartment is maintained in a substantially pressure-less state for a significant amount of time, such as 15-20 minutes, during which the vessel chamber is heated and pressurized. After the 15-20 minutes expires, the compartment is pressurized to a desired pressure to counteract the chamber pressure. Subsequently, the cure is completed within the vessel. Once the retreated tire is cured, the tread width is substantially reduced, at times by 3-4% of the original width. On narrower vehicle tires, such as 225-260 millimeters (mm) nominally wide tires, shrinkage may be at least 3-4 mm, while on wider tires, such as 390 mm nominally wide tires, shrinkage may surpass 14 mm.
Tread width shrinkage is not desired for numerous reasons. For example, shrinkage forms irregular tread elements, such as tread blocks, which detrimentally affects tread wear and tire uniformity. Also, narrower treads reduce the contact patch of a tire, and therefore, reduce grip and increase wear. Further, tread voids, such as tread grooves, are reduced to detrimentally affect wet tire performance. Accordingly, it is desirous to provide a method of curing the retreaded tire while significantly reducing the tread width shrinkage in the cured retreaded tire.